Compositor - Frameserver Input
Open a new instance of Blender.
Set the project render settings; render size, # of frames, output etc.
Switch to the compositor view.
In the compositor; select use nodes, then with your mouse over the Node editor window press "SHIFT + A" on the keyboard, and select Input -> Image. In the node that appears click Open, then navigate to the directory containing the image sequence you wish to process, and select the frames to use (images should be named as frame numbers i.e. 001.png 002.png etc)

I think this is what you are looking for.